We propose to analyze the role of triacylglycerol (TG) metabolism in mammary epithelial development and breast cancer using mice lacking DGAT1 [unreadable] (Dgat1-/-), a major enzyme that synthesizes TGs from fatty acids and diacylglycerol substrates. In normal mammary gland development, TGs are synthesized in the epithelium for milk secretion and stored in adipocytes, the most abundant cell type in the mammary stroma. Obesity, characterized by excess TG storage in adipose tissue, is under study as a risk factor for breast cancer. High intake of total fat has been associated with increased breast cancer risk in animal studies and in some nutrition and epidemiological studies in humans. However, specific mechanisms for these effects are largely unknown. In Dgat1-/- mice, mammary epithelial growth and differentiation are impaired during pregnancy. In recent years, the role of the mammary stroma in epithelial growth has been established in both normal breast development and cancer. Mammary transplantation experiments showed that the reduction in epithelial growth in Dgat1-/- mice is mediated locally by stromal effects. Based on preliminary experiments, breast tumor development is also reduced when primary breast cancer cells are injected into the mammary fat pad of Dgat1-/- mice compared to wild-types. Experiments in this proposal will explore mechanisms linking alteration of lipid metabolism in Dgat1-/- mammary stroma with both normal and malignant epithelial growth. We hypothesize that lipid metabolites or other factors are altered in mammary gland in the absence of DGAT1 and impair signaling for mammary epithelial development. We will use biochemical techniques to measure the lipid content and composition of Dgat1-/- mammary glands. We will also perform microarray experiments to analyze gene expression changes in Dgat1-/- mammary adipocytes compared to wild-types. As a complementary approach to identify altered factors, an in vitro system of co-culture of epithelium and adipocytes will be developed. To explore effects of DGAT1 deficiency in breast cancer, we will use orthotopic grafts of breast cancer cells in Dgat1-/- mice and crosses between Dgat1-/- mice and two transgenic mouse models with increased incidence of breast tumors. We will measure tumor incidence, tumor load, and morphology, and determine whether the protective effect of DGAT1 deficiency is specific for one of these tumor models. Finally, based on preliminary data showing altered distribution of leukocytes in Dgat1-/- inguinal lymph nodes, we will test the hypothesis that the immune system plays a role in tumor rejection by mice. [unreadable] [unreadable]